Hydroelectric dams are the oldest technological innovation in our countdown. During the early 1900s, 40 percent of the electricity used in the United States came from hydroelectric dams. Today, hydropower accounts for nearly a quarter of all electricity used worldwide. In addition, the physical structures themselves are marvels of human engineering and construction, drawing photographers and tourists, according to the U.S. Bureau of Reclamation.
Hydroelectric dams work by holding back massive amounts of water and allowing a limited amount to flow through the dam. The water pressure created by limiting this flow is tremendous, and hydroelectric plants harness this pressure to turn turbines attached to electric generators. As with wind turbines, the electricity generated from a hydroelectric dam is added to the electric grid associated with the dam's geographic location.
A hydroelectric dam supplies the electric grid with several hundred kilowatts to several thousand megawatts of electricity per second. The Unites States National Renewable Energy Laboratory states that the largest damns in the world can generate around 10,000 megawatts, enough for millions of people to use.
Despite its age, the future is bright for hydropower as the hydroelectric dam gets a 21st-century makeover. Researchers are finding ways to improve the efficiency and environmental impact of hydropower by improving existing dams and building new dams.